Matt, your suggestion for "monolizing" is valid but usually introduces more difficulty to the situation than necessary. I think the theory of sound cancellation will be much worse than the practice of it here -- you won't notice.

We stick with stereo, and wire them L,R,L,R, so L's are in opposite corners and R's are in the other opposite corners.

This works well if the space is used with people mostly along the walls (for instance, four outdoor speakers around a pool). If the gazebo will be used with perimeter seating, it will also work well. With this arrangement, no matter where you sit (except directly under one speaker), you hear two channels, which may not be better than mono in the particular location, but it will not be worse, so going mono is no improvement.

Using four speakers also helps eliminate what I call "The Woodstock Effect" -- some people are too close to speakers, so they go deaf, and others are too far away, so the sound is less than satisfying. More speakers mean they all can be at a lower volume yet fill the space more equally.

Does the 995's second zone output have a volume control on the 995? If it does, you only need a power amp. If it does not, you need an integrated amp or a power amp with input volume controls.